1. Field of the Invention
This invention relates to an improved catalytic structure which is especially well suited for use as a means of catalytically reducing oxides of nitrogen, specifically nitric oxide and nitrogen dioxide (herein collectively referred to as NO.sub.x), which are found in the exhaust gases emanating from an internal combustion engine. In particular, the instant invention concerns an extremely durable catalytic structure which is capable of catalytically reducing NO.sub.x in a hydrogen and/or hydrocarbon atmosphere substantially without the generation of ammonia which comprises an expanded, heat resistant metallic substrate having deposited on the surface thereof an alloy of a nickel base material and copper with the weight ratio of the nickel base material to copper being at least equal to, but preferably greater than, about 9 to 1.
2. Description of the Prior Art
Many undesirable gases are found in the exhaust stream emanating from the internal combustion engine. Some of the most harmful and annoying of these gases are carbon monoxide, unburned hydrocarbons and the various oxides of nitrogen.
Presently, much effort is being directed toward the removal of carbon monoxide and unburned hydrocarbons by thermal or catalytic oxidation and to the diminution or elimination of the oxides of nitrogen by catalytically induced reduction. Specifically, with regard to the various oxides of nitrogen, the present desire is to eliminate at least 90 percent of the NO.sub.x found in the exhaust gases issuing from the typical internal combustion engine, using the 1971 U.S.A. automotive vehicle as the base.
To date, various catalysts have been suggested as a means of catalytically reducing NO.sub.x to less harmful substances. For example, in U.S. Pat. No. 3,565,574 it is suggested that metals or alloys of metals chosen from the metals set forth in Groups I-B and IV to VIII of the periodic system are suitable catalysts for the catalytic reduction of NO.sub.x. It is particularly pointed out therein that a Monel type alloy containing 66 weight percent nickel and 31.5 weight percent copper, plus 2.5 weight percent impurities is a very effective catalyst for the reduction of NO.sub.x gases found in exhaust gases emanating from an internal combustion engine, especially when operating at temperatures in excess of 1200.degree. F.
While catalysts of the foregoing type have been generally effective as a means of catalytically reducing NO.sub.x, it has now been discovered that certain catalysts of the Monel type, i.e., those which contain a high weight percentage of copper, suffer from the disadvantage that in a hydrogen and/or hydrocarbon containing atmosphere a significant amount of the nitrogen component of the NO.sub.x is reacted with hydrogen to form ammonia. Accordingly, when such a NO.sub.x catalyst is used in a catalyst system wherein the exhaust gases are first brought into contact with a reduction catalyst and then into contact with a separate oxidation catalyst (that is, in a dual catalyst system), the ammonia generated by means of the reduction catalyst is then oxidized by means of the oxidation catalyst to reform undesirable NO.sub.x gases.
In summary, it can be said that the prior art has provided nickel-copper, Monel type catalysts which are capable of catalytically reducing NO.sub.x but it has failed to provide a catalyst which consists essentially of a nickel base material and copper having a high degree of durability which when used in a hydrogen and/or hydrocarbon containing atmosphere does not produce a significant amount of ammonia.